A known technique for performing multi-band antenna matching tunes the antenna structure itself. However, this can become a complicated process if the antenna has many frequency bands. In addition, multiple antenna feeds are used rarely because of the poor isolation between ports.
A persistent problem with mobile station antennas is the need to decrease the antenna volume while covering more frequency bands. It is well known that, especially in the GSM850/900 bands, the chassis of a mobile station may function as the main radiator. The antenna element can be understood as a matching circuit and a coupling element between the port of the antenna and the chassis of the mobile station. In order to be able to implement a wideband antenna in a small volume, it is necessary that the antenna element couples strongly and efficiently to the characteristic wavemode of the chassis.
It can be determined that the strongest coupling to the chassis wavemode can be achieved at the corners and shorter ends of the internal ground plane. A strong coupling to the chassis wavemode requires the maximum of the electric field of the antenna element to be located near the maximum of the electric field of the chassis. In addition, the electric field strength all around the antenna element should be as high as possible, i.e. the volume of the antenna should be used efficiently. In this respect, the structure of one of the most commonly used internal mobile station antenna, the PIFA, is not optimal. Near the shorting pin of the PIFA, the voltage and thus also the electric field strength is low. Also, the requirement of self-resonance is a limiting factor for an antenna designer for two different reasons. First, due to the self-resonance, the space requirements of the PIFA at low frequencies, e.g. at the GSM850/900 bands, are rather high. As a consequence, some type of meandering of the antenna element is needed in order to reduce its total volume. Second, owing to the meandering at the lower frequencies, it becomes difficult to optimally shape the PIFA according to the high-coupling locations of the chassis.
It is believed that stronger coupling to the chassis wavemode has been primarily achieved by moving the antenna element (PIFA) partly over the edge of the chassis. Multi-band/multi-resonant mobile station antennas have traditionally been implemented using multi-resonant antenna elements and parasitic resonators.